function function_exists(func_name) {
  //  discuss at: http://phpjs.org/functions/function_exists/
  // original by: Kevin van Zonneveld (http://kevin.vanzonneveld.net)
  // improved by: Steve Clay
  // improved by: Legaev Andrey
  // improved by: Brett Zamir (http://brett-zamir.me)
  //   example 1: function_exists('isFinite');
  //   returns 1: true

  if (typeof func_name === 'string') {
    func_name = this.window[func_name];
  }
  return typeof func_name === 'function';
}